The present technology relates to a process for the polymerization of olefins carried out in the presence of a supported antistatic compound. The technology also relates to a supported antistatic compound, a process for its preparation and its use in a process for the polymerization of olefins.
In continuous polymerization processes, for example in gas-phase processes for the polymerization of olefins, there is the need to “face up” to the formation of polymer agglomerates in the polymerization reactor, which are capable of introducing many negative effects in such processes. For example, they can disrupt the discharge of polymer from the reactor by plugging the polymer discharge valves. Furthermore, polymer agglomerates may cover the fluidization grid of the reactor, resulting in a loss in fluidization efficiency.
It has been found that the presence of fine polymer particles, or fines, in the polymerization medium favors the formation of polymer agglomerates. These fines may be present as a result of introducing fine catalyst particles or breakage of the catalyst and polymer particles within the polymerization medium. The fines are believed to deposit and electrostatically adhere to the inner walls of the polymerization reactor and the equipment for recycling the gaseous stream, for example, the heat exchanger. If the fines remain active, the particles will grow in size, resulting in the formation of agglomerates, which may also be caused by the partial melting of the polymer itself. These agglomerates when formed within the polymerization reactor may be in the form of sheets. Agglomerates can also partially plug the heat exchanger designed to remove the heat produced during a polymerization reaction.
Several solutions have been proposed to resolve the formation of agglomerates during gas-phase polymerization processes such as the deactivation of the fine polymer particles, the control of the catalyst activity and the reduction of the electrostatic charge by introducing antistatic agents inside the reactor.
EP 0359444 describes the introduction of small amounts of an activity retarder into a polymerization reactor in order to keep the polymerization rate or the content of transition metal in the polymer produced constant without forming agglomerates.
U.S. Pat. Nos. 4,803,251 and 5,391,657 describe methods for reducing polymer sheeting by adding to the reactor additives that generate positive or negative charges depending on whether the electrostatic level detected in the reactor is negative or positive, respectively. In U.S. Pat. No. 5,391,657, silicon dioxide (SiO2) is mentioned among the possible negative charge generating additives.
EP 0560035 discloses a polymerization process in which an anti-fouling compound selected from alkyldiethanolamines is used to eliminate or reduce the build-up of polymer particles on the walls of a gas-phase polymerization reactor that may be fed at any stage of the gas-phase polymerization process in an amount greater than 100 ppm by weight with respect to the produced (co)polymer.
WO 2007/041810 discloses supporting an olefin polymerization catalyst together with a polysulfone antistatic compound on a porous metal oxide and using such supported antistatic catalysts in olefin polymerization.
WO 2012/041810 and WO 2012/041811 disclose methods for feeding antistatic components to a polymerization reactor by preparing a catalyst suspension incorporating the antistatic components and successively transferring the catalyst suspension to the polymerization reactor.
A detrimental side effect of the use of antistatic compounds in polymerization processes is that they also act as “catalyst poisons” and therefore, even if used in small amounts, reduce polymer yields. None of the previously described methods has successfully addressed the problematic formation of polymer agglomerates in the reactor while at the same time minimizing negative effects on catalyst yield, which may advantageously be achieved by the process of the present technology.